Magnetic tape comprises a medium for storing large amounts of data, and typically comprises a plurality of parallel data tracks that extend longitudinally along the tape. A tape head is employed for reading and/or writing data on the data tracks, and is typically shared between various data tracks or groups of data tracks, and is moved between data tracks or groups of data tracks in the lateral direction of the tape. The tape head typically comprises a number of separate data transducers which read and/or write data with respect to a number of parallel longitudinal data tracks. Servo systems are provided to position tape heads laterally to position data transducers over the desired data tracks and to then maintain alignment of the data transducers with respect to the desired data tracks, as the magnetic tape is moved longitudinally with respect to the tape head. The tape head is provided with one or more separate servo heads, which are offset from the data transducers, so as to maintain tape head alignment by track following a servo band of the magnetic tape. A servo head is guided along any of several paths within the band, called “index positions”, and the tape head is repositioned laterally within a servo band so that the data transducers access different data tracks. The servo bands may be continuously variable laterally to provide a servo signal which varies continuously as a function of the lateral position of the servo head, allowing a calculation of a Position Error Signal, or “PES”.
One type of servo system for magnetic tape media is one in which a plurality of separate servo bands are laterally positioned on the magnetic tape media. Each of the servo bands provides the servo guidance for a group of data tracks, and the servo transducer of the tape head is repositioned laterally within a servo band so the data transducers access different data tracks, and is repositioned laterally to another servo band to access still further data tracks. In one example, the servo bands are spaced apart and the data tracks are located between the servo bands. To insure that the servoing is precise, two servo transducers may be provided at either end of the tape head, straddling the data transducers. The lateral position information may be obtained from either or both servo bands. The servo bands may be encoded with essentially identical patterns for determining lateral position, such that the bands are substantially interchangeable from the point of view of calculation of the PES, or both may be used (e.g. averaged).
The lateral positioning of the tape head is typically accomplished by actuators, which may have mechanical or electromechanical components. Once the proper lateral positioning of the servo head of the tape head over a servo band has been accomplished, as the servo information being sensed by the servo head indicates, minor adjustments of the tape head may be made to track follow lateral movement of the tape or of the servo bands on the tape.
However, the lateral repositioning of the tape head between the servo bands is typically conducted by a coarse actuator which operates in open loop without feedback. Thus, as the tape head is repositioned between the servo bands, there is no feedback from the servo information to indicate that the tape head has actually moved from one servo band to another servo band. Hence, at the supposed completion of the lateral movement, the servo head of the tape head may be positioned over data tracks instead of a servo band, and the lateral position of the tape head is unknown, and information about the lateral position is “lost”, as defined herein. Further, servo information is lost during movement of the servo head between servo bands since the servo head is unable to provide servo information.
One way of determining where the tape head is positioned, is to provide a separate “independent” sensor, for example, that determines the approximate lateral position of the head with respect to the tape. Such an independent sensor may comprise a coarse optical sensor that measures the physical position of the head. Such a coarse sensor cannot typically be used for track following, but provides a backup to the actual servo system should coarse positioning fail to place a servo head of the tape head over a servo band. Such extra sensors add cost to a tape drive, which is always undesirable, if the extra cost can be avoided. Another example is to arbitrarily move the tape head laterally in hopes that the servo head will meet a servo band. It is possible that the tape head has moved to the edge of the magnetic tape and the arbitrary movement will either move the tape head off the magnetic tape, or into an overshoot stop that may be provided.